Elimination of distortion in electron discharge devices



Jan. 17, 1939. c. w. HANSE L ELIMINATION OF DIISTORTION IN ELECTRON DISCHARGE DEVICES Original Filed May 21, 19:1 2 Sheets-Sheet -1 m w ran/m:

REES un I 400 57/0 VOLT/16E INVENTOR CLARENCE W. HANSELL BY LQ ATTORN EY Jan. 17 1939, E c. w. HANSELL 2,144,492

' ELIMINATION OF DISTORTION IN ELECTRON DISCHARGE DEVICES Original Filed May 21, 1931 I 2 Sheets-Sheet 2 wzi vvvvvvv HMPL 075a 3'0 i .54 HIGH FeE UENcYSM/ms INVENTOR MOOULHT/ONSOl/ECE CLARENCE W. HANSELL 'H'ggMjw n/ ATTORNEY Patented Jan. 17, 1939 PATENT .oFFIcE MINATIO OF DIS T ON IN iicrno DISCHARGE DEVICES :Glare c W. Hansen, Port Jafficrsca r to Rad o. q n a s l ration of Delaware Application May 21, 193 1,--Serial No. 538,958

" Renewed January 21;:1937

mai si i te i This invention has as its principal object the provision of methods and means for the elimination of distortion in electron discharge device amplifiers and especially the distortion due to the drooping of the characteristic curve of an amplifier caused by grid rectification.

A linear characteristic for the output of an electron discharge'device amplifier relative to its input may readily be obtained when the output of the amplifier is' kept within approximately half of its commercial rating for high eff ciency continuous Wave operation. However, beyond that point, the ratio of output to input for the amplifier is no longen a constant and the characteristic curve which represents the-ratio between the input and 'output of the amplifier droops beyond the half rating'value. Inthe case of telephonic cornmunication or multiplex coinmunication, this distortion limits a transmitter output to approximately half of' what its rated output would befor continuous'wave-telegraphy, making' a .great reduction in power and transmitter 'efiiciency as Well as increasing cost of construction-and operating a expenses for transm'i'ssi on 'o'fmodulatedsignalsof desired power Where the amplitude of the output must raunfully follow the wave 1 shape of the modulat on energy.

'The drooping 'characteristic of an amplifier may be attributed to severalcauses, thjno'st important of which is' decrease in effectivegrid resistance starting with grid rectification a Relative tooth'er causes of distortion'such as saturation of vacuumftubesused for an amplifier, grid rectification presents a difiicult problem inasmuch as it causes a discontinuous break in the characteristic amplification curve of the amplifier "That is, in the caseoffsatur'ation, the saturation characteristic of tubes is'fairly continuous and can be balanced out magma extent by adjusting earlier amplifier stages fsofthat they produce a curvature opposite o thesaturation curvature to be corrected;

i-leretofore; in order to reduce the efiect of grid rectificatiomthe 'input circuit of the" amplifier has been artificially loaded withresistances so that the increased losses produced by the grid rectification are'sinall in comparison" with' the losses "throughout the loaded'input circuit. 'Consequently; the change in effectivegrid resistance,

due to rectificationghas a relatively small efiect upon theivoltageregulation of the inputcircuit and, therefore, upon the-curvature .ofith'e amplifier characteristicz: i V

' But such a 'loadingiinethod, iniorderatabeipracdroop. lEriomithat ;ti1"ne;onrthe lastireferred to tically effective, necessitates a large ratio between radio f requency losses in theinput 01 grid excitation circuit' -relative :to the grid losses, entailing a marked reductionlinipower, Lefilciency and amplificatio'n in the amplifier. As a. result of this, J! for a given output, an'increasednum'ber of stages are needed having concomitant thereto, disadvantages of increasedv construction and operating costs to :say nothing of increased unreliability of an installation. V

T0=bvercome theiorego'ing difficulty is another object of theipr'esent invention and according to it, it-isaccom'plished byieeding energy to anamplifier having ianfiin'desired characteristic through a plurality of electron discharge devices. One-of the devices is so designed oradjusted that ;it ,continually :feeds power into 1 the amplifier, the .ratio of thelpowerriedlto the power supplied to ,thevdevicelbeing substantially constant. However, taccordingtolithe present invention, another ofithe 2 couplingelectron idischarge'devices is so designed or isoizadjusted orzbia'sed that it delivers .little .01: no output :uritil the excitation .of the power amplifier is :such that its characteristic begins to electron-discharge;device wi1l; addimoreiand more powerito the output :of xthe first electronidischarge device ithereby contacting .the .ldroop- .in the power iamplifier characteristic. aresult of this structure a power amplifier is made capable .ofigiving va much higher output before the distortionlimit islreached. I

i-illf'irlesired theliitwo coupling :electron discharge devices may be .tmountedowithinna single levacuated envelopemr may be combined intooneset of elements difflerent'areas of which have different'; characteristics. For example, one portion of cthe area of .aithreetelement vacunmfiube may have. a coarse :grid .and another :area-a fine grid such that :ztheiarea:.with the fine grid ,-.does"' not begin Vito have (an electron discharge until .the conditions tare reached uwhere .alciorrectionof the characteristic is desired. Ifubes having Ldifie'rentlspa'cin'gs :between portions of theielements mayibeiiused .or:there may be differences ainiboth theigridmesh andlthes'pacingsq rlI'o more :fully understand Lthe present invention, sreferenc'e istmade to .the accompanying drawings which; it :lto be clearly understood, is giJZQIISOII-YiBYZWay ::of v illustration .ofsthe :present invention-land lwherein I zFigurei'Ll is ;a graph illustrating the drooping characteri ti 50f a commerci :PQW am lifi he idmc ihe i c ne .sha nreximete 559% 2. terated Ro en-o t ut. i I

Figure 2 illustrates a system for correcting the droop of the amplifier whereby its linear characteristic is continued past the point where ordinarily drooping would 'occur due to grid rectification, V

Figure 3 is a modification of the invention illustrated'in Figure2, and,

Figure 4 shows the efiect upon the characteristic which is obtained by paralleling or using in i a single pushpull amplifier stage, in any ordinary amplifier design, two commercialtubes-having different amplification constants but like circuit and potential adjustments.

mercial power amplifier and the input voltage applied thereto. From inspection it is clear that at point 4, where grid rectification occurs, the characteristic of the amplifier is no longer a straight line but becomes curved; resulting in modulation distortion and, in the event that the amplifier-is usedin a transmitter for telephone or multiplex telegraph communication purposes, this will tend to limit the amplifier output, re-

quiring, for a given strength of signal, additional 7 amplifiers, thereby increasing the cost of installation as well as operating costs therefor.

. To operate an amplifier such that it will have a straight line characteristic beyond the point where grid rectification occurs, it is proposed, ac-

cording to the present invention, toyary the input to the amplifier suchthat the curvature thereof is compensated and so that, referring to Figure 1, the characteristic'of the amplifier con tinues along the dotted straight line 6 beyond thepoint where gridrectiflcation occurs.

Referring to Figure 2 which illustrates apparatus for. carrying into effect the foregoing desirediresult, undulatory currents from a suitable source 8 are fed to an amplifier l0 here in the form of a screen grid tube with a tuned output circuitand thence through electron discharge devices I2, [4, to a power amplifier. lficomprising pushpull connected capacity neutralized electron discharge devices I8, 20. The screen grid of. tube I0 is supplied with direct current by a resistance I 1. connected to the direct current potential circuitffor the anode of II]. A bypass condenser .3 may be. connected between the screen grid and cathode of ID...

To neutralizeor correct the drooping characteristic of the power amplifier l6, tube l4, by virtue of its coupling through lead 22 to amplifier i0 .is givena higher excitation than tube l2; but, its control electrode 24 is more negatively polar! ized or. biased from potentiometer 26 and tappingconductor 28 so that it is cut oif or rendered non-. conductive until the output of amplifier l6. begins to droop as illustrated by point 4 of Figure 1. .The excitation voltage amplified by'tube II! at which-the output of amplifier l6 starts to i droop is sufi'icient to overcome the negativebias on the grid'of tube l4 and the output of electron discharge device I4 isadded to that of tube I2 and their combined outputs give a greater excitation to amplifier l6 than tube l2 alone could neutralized or as illustrated they may be of the screen grid type, the screen grids being suitably biased by the action of resistors l I connecting the respective screen grids to the respective anode polarizing leads for the tubes. I prefer to use the screen grid tubes because, over the operating range their plate potential variations have less effect upon the anode currents than is the case with three electrode tubes and the efiect of tube 14 upon the radio frequency anode voltage of tube l2. has less tendency to reduce the output currentof tube l2. Consequently the two output currents remain more unafiected by one another Turning to Figure 1, the solid line 2 indicates the relation between the power output of a comand tend each to add its portion of output current in the manner desired. The screen grids,

of course, 1 have been grounded for radio frequency potentials by the use of by-pass condensersillustrated diagrammatically at [3 and, radio frequency flow through potentiometer 26 is prevented by the action of radio frequency chokes Rectangle 8 not only diagrammatically represents a source'of undulatory current but indicates as well and diagrammatically also, means for modulatingthe energy from the source. In the event that itis desired to introduce modulation into a stage immediately preceding the power amplifier and yet have a straight line characteristic for the same beyond the point where grid rectification occurs, the arrangement shown in,

Figure 3 may be utilized to good advantage; In Figure 3, energy from a source 30, of undulatory electrical currents, is fed to a pair of screen grid electron discharge devices 32, 34.

Modulating energy 7 of relatively low frequency from a source 36 is fed to a modulation amplifier 38v whose anode together with the anodes of screen grid tubes 32, 34 is fed through a choke coil; 40 with suitable anode polarizing potentials supplied by leader conductor 42. V

The control electrode 44 of screen grid tube 32, the screen grid 46 through the action of resistance 48 coupling itgto the anodepotential source, and the anode of tube 32 are so polarizedas to give'a radio frequency output current substantially proportional to. the applied input voltage over the desired range ofoperation. 7

Tube 34 difiers in polarization in that the screen grid'ill thereof is biased through a potentiometer 52 so that it has'a negative or low positive potential thereon until modulation of the anode voltage by the high signal .on tube 38 makes it more positive. Thistakes place at and to cause distortion in thepower amplifier 54. In 7 other words, there is little output from tube .34 until the droop of the amplitude characteristic of the power amplifierdesignatedin Figure 1 by point 4 is reached. At that time tube 34 will have higher potentials on its screen grid and anodeand will begin toadd power to the output of'tube 32and, by suitable adjustment of circuit elements, will compensate for the curvature in the amplifier characteristic of power amplifier 54 which may consist of one or more amplifier stages.

Rather than polarize electrodes of a plurality of; tubesto different values such that the output of. one adds to the output of the other only after a predetermined operating point is reached,'the same result may be obtained by combining two tubes having difierent characteristics 'orinother words; different amplification constants. These different characteristics may be due to any number Jon-reasons; for example, by using diiierent grid mesh and: possibly difierent spacings'between control electrodes such as the grids and cathodes or filaments. Two such tubes used in asingle or the same amplifier stage either in parallel or in opposite sides of a pushpull circuit, would give a desired characteristic such as disclosed heretofore.

Thus, referring to Figure 4, a tube havingthe characteristic given by curve Ia may be combined either in pushpull or in parallel with a tube having the characteristic indicated by the solid line 3a of Figure 4; The resultant characteristic of the combination of tubes polarized alike but having different amplification characteristics as indicated would be given by the dotted curve 4a which, of course, has a desired output characteristic relative to the input.

This arrangement may be extended not only to a pair of tubes having different characteristics, but to groups of tubes of differing characteristics whereby the combination results in a more desirable characteristic. To provide an added adjustment, the difierent tubes or groups of different tubes may be polarized differently.

It should be clear that although only two schemes have been illustrated and described for carrying out the foregoing thought of having an auxiliary tube added to the output of another tube only after a certain output amplitude is reached and then in such proportion as to obtain the correction of an unfavorable amplifying characteristic, many other detailed circuits, in the light of the foregoing teachings, may readily be set up by those skilled in the art, and consequently the present invention is not to be limited in scope by the modifications shown but is to be given a scope commensurate with that indicated by the appended claims.

7 Having thus described my invention, what I having a drooping plate current grid potential characteristic, an electron discharge device connected to said amplifier for supplying signal energy to be amplified to said amplifier, and an auxiliary electron discharge amplifying device for supplying additional signal energy to said amplito said amplifier during the time that the amplicharge devicesv feeding energy from said source to fier is operated on a portion of its characteristic which normally droops due to grid rectification whereby said amplifier continues to have a linear output with respect to input for large input values.

3. In apparatus for the amplification of undulatory electrical currents, a source of undulatory electrical currents, an amplifier for amplifying currents from said source, a pair of electron discharge devices coupling said source. and said amplifier, one of said electron discharge devices continually feeding energy from said source to said amplifier and the other of said electron dissaid amplifier after the output of said amplifier would normally droop due to grid rectification whereby said amplifier continues to have a linear output with respect to input for large input values.

4. In apparatus for the amplification of undulatory electrical currents, a source of currents to be amplified, an amplifier, having a successively linear and drooping characteristic, for amplifying the currents, a pair of pushpull connected electron discharge devices for coupling the source to said amplifier, and means for biasing one of said electron discharge devices to give a relatively of undulatory electrical energy, a pair of pushpull connected electron discharge devices coupled to said source, said pair of devices having their cathodes connected together, their control grids connected in phase opposition and their anodes connected in phase opposition, an amplifier coupled to said pushpull connected electron discharge devices, and means for preventing fiow. of energy through one of said pushpull connected electron discharge devices until the output of said amplifier reaches a predetermined value.

6. In apparatus for the amplification of undulatory electrical currents, a source of undulatory electrical energy, an electron discharge device coupled to said source, an amplifier coupled to said electron discharge device, said electron discharge device feeding at all times energy from said source to said amplifier, another electron discharge device coupling said source and said amplifier, said last mentioned electron discharge device being coupled to said source so as to have greater excitation than said first mentioned electron discharge device, and means for biasing said last mentioned electron discharge device so that it feeds energy from said source to said amplifier only after the output of said amplifier has reached a predetermined value.

7. In apparatus for the amplification of undulatory electrical currents, a source of undulatory currents, an amplifier for amplifying the currents, a pair of electron dischargedevices coupling said source and said amplifier, said pair of devices having their output electrodes connected I to a common output circuit, which circuit forms the input circuit for said amplifier, and means including a source of direct current potential for biasing one of said electron discharge devices so that it does not feed energy from said source into said amplifier until after a predetermined output of said amplifier is reached, at which time the output of said biased electron discharge device is added to the output of the other of said pair of electron discharge devices to cause an increased input to said amplifier.

8. In a system for amplifying alternating currents, a source of alternating currents, an amplifier for amplifying the alternating currents, and means for coupling said source to said amplifier, said coupling means comprising a pair of electron discharge devices having their output electrodes coupled to said amplifier and their input electrodes connected to said source of alternating current, said connection being such that the input electrodes of one of said electron discharge devices is subjected to higher amplitude alternating potentials'from said source than the other.

9. An amplifying system for amplifying alter- 75 nating currents comprisinga sourceof alternating currents anda coupling system for'icouplin'g the source to said amplifier, said coupling system comprisinga" pair of electron discharge devices each havingan anode a cathode a control electrode and a screen grid, 2. common alternating current circuit connected tothe anodes of the devices forming said coupling system, means for subjecting one of said screen grids to a more positive unidirectional potential than the other and means for feeding energy from said common cir- -cuit to said amplifier.

10. In combination, a source of alternating currents to be amplified, a first electron discharge device amplifier coupled to said source, said discharge device having a tuned output circuit,. a second high power electron discharge device amplifier and means coupling the tuned output circuit of said first mentioned discharge device amplifier to said high power electron discharge device amplifier, said means comprising a pair of electron discharge deviceshaving like electrodes subjected to difierent uni-directional polarizing quency circuits connected to said input and out-.

put electrodes so that said electrodes fluctuate in phase opposition, and means for subjecting said additional ielectrodesito di fferent uni-directional ipolari zing :potentials'wher'eby one of said devices does notifeed appreciable energy into said power ampli'fier until a predetermined amplitude input to sai'd devi''es is reached.

IZZFIn atranSmitting system, a high frequency circuit, means for causing modulated high frequency wavesto flow in said circuit, a pair of vacuum tubes in said circuit, means for subjecting the input electrodes of the tubes of said pair of tubes to relatively different direct current poten tials, meansfor subjecting the control electrodes 'of said tubes of said high frequency circuit to modulated radio frequency potentials of different values, the value of the radio frequency potential supplied to the grid of the tube having the least positive direct current potential being greater than the value of the radio frequency potential supplied to the grid of the other tube, a common output circuit for said tubes, and means for combining the outputs of said tubes in said common output circuit. v

13. In an amplifying system, a source of alternating currents to be amplified, a power amplifier to amplify currents from said source, means for coupling said source to said power amplifier, said means comprising a'pair of electron discharge 7 devices having a common high frequency input circuit and acommon high frequency output circuit, means for subjecting electrodes of said devices to uni-directional potentials such as to render one substantially less conductive than the other,'and means for subjecting said electron discharge device of lesser conductivity to higher alternating potential from said source relative to the potential supplied from said source to the other device of said pair of devices.

, CLARENCE W. HANSELL. 

